EVOLUTION ioi 



progress as exhibited in the vertebrate kingdom; 

 the very plasticity of these creatures has thwarted 

 progressive development, as we understand the 

 phrase, and they readily retrograde, or branch off, 

 into bypaths. Nevertheless, progressive develop- 

 ment is traceable in certain characters, while in 

 other conspicuous features the action of environment 

 or individual requirement alone seems responsible. 



It will suffice to take certain leading features and 

 organs to summarize what is known concerning 

 them, and to point out what conclusions, if any, can 

 be based upon them. 



The tidal zone was in all probability the cradle 

 of the race, although it has been argued by some 

 with much show of reason that the earliest forms 

 were pelagic. From the shore the various members 

 betook themselves, mostly to deeper and deeper 

 water on the one hand, but also, though perhaps 

 more tentatively and gradually, to fluviatile and 

 terrestrial conditions on the other. 



Now, the first requirement of a soft-bodied animal, 

 and especially of one considered by its fellows to be 

 good eating, is protection. In early days, however, 

 enemies were far fewer than now, and it was rather 

 from the force of the elements that preservation was 

 needed. 



This first requirement is supplied by the shell, 

 and all three types, Univalve, Bivalve, and Multi- 



